
Δ9-Tetrahydrocannabinol (THC, I) is the primary active ingredient of the plant Cannabis sativa (marijuana) and is responsible for the majority of the pharmacological effects. People have utilized the plant (that includes numerous cannabinoids) since ancient times for medicinal purposes as well as for its intoxicating properties. While marijuana is primarily known as an abused drug, we believe that there are important pharmacological properties of the active component THC that could be directed to specific therapeutic effects, given the appropriate delivery mechanism. To date, the most promising clinical applications approved by the Food and Drug Administration (FDA) are for the control of nausea and vomiting associated with chemotherapy and for appetite stimulation of AIDS patients suffering from anorexia and wasting syndrome[1, 2].
THC, however, demonstrates other biological activities which lend themselves to possible additional therapeutic applications. These include glaucoma[3], migraine headaches[4, 5] spasticity/epileptic seizures[6, 7], anxiety[8] and chemical dependence withdrawal symptoms. Also, more recently, THC is becoming increasingly recognized as an analgesic[1, 2, 6, 7]. Due to these promising biological activities, THC has potential for multiple medicinal uses.
Challenges in Systemic Delivery of THC:
Parenteral formulations researched include an intramuscular preparation[9] and an intravenous dosage form (neither of which have been approved by FDA). Injectables are inundated with the problems of being invasive and requiring professional assistance, and therefore in many cases preclude self medication. In addition, these parenteral routes are inherently subject to abuse.
Thus, the search for a non-parenteral delivery system for THC continues. The physicochemical characteristics of THC, like many other lipophilic agents, present major challenges to drug delivery scientists. The log P (log octanol/water partition coefficient) value of THC is around 4.0 indicating that it is a highly lipophilic compound. Consequently, THC's solubility in the gastro-intestinal fluids and partitioning from the intestinal membranes into the systemic circulation would be severely limited. Additionally, THC is rapidly metabolized by the hepatic CYP 450 enzymes to the 11-hydroxy metabolite (11-OH-THC), which is responsible for the undesirable side effects of THC[9, 10]. The blood plasma levels desired are in the range of 10 ng/ml—enough to provide a therapeutic effect without the production of a significant “high” (>100 ng/ml)[11, 12]. Poor gastro-intestinal stability of THC further hinders oral absorption. These factors act in conjunction to minimize systemic availability of THC following peroral administration, the most preferred route of administration, and forms the basis of one of the main issues brought into public debate by medicinal marijuana proponents—the fact that the currently available soft gelatin formulation is expensive and lacks consistency in its therapeutic effects and pharmacokinetic profiles. It is significant to note, however, that the only THC dosage form currently approved by FDA is the oral, soft gelatin capsule (Marinol®).
Considering the challenges in intestinal absorption and metabolism, attempts have been made to deliver THC through nasal as well as inhalation routes[13-15]. In a recent phase I pharmacokinetic study[16] performed by GW Pharmaceuticals, UK, Cannabis-based extracts were tested by three different routes of administration via; sublingual, buccal and oropharyngeal. The formulation was administered in the form of sublingual drops as well as a pump action sublingual spray (PASS). In this study, it was reported that buccal administration of the PASS test treatment resulted in a later Tmax but greater Cmax when compared to the sublingual and oropharyngeal routes. However, AUC was reported to be greatest following the oropharyngeal route. The lower bioavailability (measured in terms of AUC) following buccal administration, as compared to the sublingual and oropharyngeal routes, is most likely related to the difficulty of spraying onto the inside of the cheek and subsequent loss of the spray.
Although promising, the nasal and oropharyngeal routes are burdened with problems. Potential irritation and the irreversible damage to the ciliary action of the nasal cavity from chronic application of nasal dosage forms, and large intra- and inter-subject variability in mucus secretion in the nasal mucosa that could significantly affect drug absorption from this site. Also, the inhalation route of administration possesses high abuse characteristics. In addition, spray formulations of THC have been shown to have a dosage form-related adverse effect of throat irritation[16]. Other non-parenteral routes examined include sublingual[17], rectal[17-20] and vaginal[21].